[c++] Makefile: How to correctly include header file and its directory?

I have the following makefile:

CC=g++
INC_DIR = ../StdCUtil
CFLAGS=-c -Wall -I$(INC_DIR)
DEPS = split.h

all: Lock.o DBC.o Trace.o

%.o: %.cpp $(DEPS)
    $(CC) -o $@ $< $(CFLAGS)

clean:
    rm -rf *o all

This makefile and all three source files Lock.cpp, DBC.cpp, Trace.cpp are located in the current directory called Core. One of the source file Trace.cpp contains a line that includes a header file outside the current directory:

//in Trace.cpp
#include "StdCUtil/split.h"

The header file split.h is located at one level above the current directory and then in the subdirectory called StdCUtil. So that's why I added INC_DIR = ../StdCUtil in the makefile. The overall directory structure looks like the following:

root
  |___Core
  |     |
  |     |____Makefile
  |     |____DBC.cpp
  |     |____Lock.cpp
  |     |____Trace.cpp
  |
  |___StdCUtil
        |___split.h

But when I make it, it gives me the error:

Trace.cpp:8:28: fatal error: StdCUtil/split.h: No such file or directory
 #include "StdCUtil/split.h"
                            ^
compilation terminated.
<builtin>: recipe for target 'Trace.o' failed

Why this doesn't find the header file split.h even if I specify the INC_DIR in the makefile? How to correct this?

Try INC_DIR=../ ../StdCUtil.

Then, set CCFLAGS=-c -Wall $(addprefix -I,$(INC_DIR))

EDIT: Also, modify your #include to be #include <StdCUtil/split.h> so that the compiler knows to use -I rather than local path of the .cpp using the #include.

This is not a question about make, it is a question about the semantic of the #include directive.

The problem is, that there is no file at the path "../StdCUtil/StdCUtil/split.h". This is the path that results when the compiler combines the include path "../StdCUtil" with the relative path from the #include directive "StdCUtil/split.h".

To fix this, just use -I.. instead of -I../StdCUtil.

The preprocessor is looking for StdCUtil/split.h in

• ./ (i.e. /root/Core/, the directory that contains the #include statement). So ./ + StdCUtil/split.h = ./StdCUtil/split.h and the file is missing

and in

• $INC_DIR (i.e. ../StdCUtil/ = /root/Core/../StdCUtil/ = /root/StdCUtil/). So ../StdCUtil/ + StdCUtil/split.h = ../StdCUtil/StdCUtil/split.h and the file is missing

You can fix the error changing the $INC_DIR variable (best solution):

$INC_DIR = ../

or the include directive:

#include "split.h"

but in this way you lost the "path syntax" that makes it very clear what namespace or module the header file belongs to.

Reference:

• C++ #include semantics

EDIT/UPDATE

It should also be

CXX = g++
CXXFLAGS = -c -Wall -I$(INC_DIR)

...

%.o: %.cpp $(DEPS)
    $(CXX) -o $@ $< $(CXXFLAGS)